fix: null check cards on layout init

[pedrolamas]

We have had a few reports from users that Fluidd stops working after editing the dashboard layout.

It seems that somehow the layout data is getting incorrectly recorded in the database with null values for some cards.

Though I don't yet know the reason for this happening, the proposed fix here is to filter these null values out before we attempt to load the layout.

Fixes #1473

[pedrolamas]

Initial fix didn't solve the problem as the null value filtering needs to be done a bit earlier, so I moved the code above.

[matmen]

I won't have access to a machine running fluidd for the next couple of days, so I can't test it functionality wise, but the code LGTM